Effects of woody encroachment on savanna nitrogen dynamics: Combining biogeochemistry and remote sensing (Texas).

摘要

Woody encroachment, the increase of woody plant density relative to herbaceous vegetation, has contributed to documented biophysical and biogeochemical changes world-wide, and locally in the southwestern U.S. In North Texas rangelands, encroaching mesquite (Prosopis glandulosa var. glandulosa ), a known nitrogen (N)-fixing species, has caused changes in aboveground biomass. However, the impacts of woody encroachment on N cycling have not been well studied, despite the central role that N dynamics play in controlling carbon (C) cycling and many other ecological processes across all spatial scales from microbial soil processes to global plant productivity. Airborne remote sensing is arguably the only approach available to develop a spatially-explicit understanding of ecosystem processes. The main goal of this research was to determine whether remotely sensible parameters of vegetation structure could be used to quantify biogeochemical changes in N at the local, landscape and regional scale.; To accomplish this goal, I first characterized the impact of woody encroachment on soil nitrogen oxide (nitric-NO and nitrous-N₂O oxide) emissions. I examined biotic (vegetation type and soil organic and inorganic N dynamics) and abiotic (soil moisture, temperature, and soil texture) controls over soil NO and N₂O emissions across a gradient of aboveground (AG) Prosopis biomass growing on two soil types.; I concluded that mesquite encroachment in these grasslands increased NO emissions in a spatially explicit manner influenced by the AG biomass and soil type, which was then temporally mediated by temperature and secondarily by precipitation.; Based on these results, I combined hyperspectral remote sensing and field measurements to quantify spatial patterns and to estimate regional fluxes of soil NO emissions across 120 km2 of semi-arid rangeland in North Texas. This analysis captured the high spatial variability of NO emissions as they co-varied with vegetation cover and soil type across the region. I concluded that relationships between NO emissions and remotely sensed structure and composition are advantageous for quantifying NO emissions at the regional scale. Linking emissions rates to remotely-sensible vegetation parameters also provided a means to quantify the role of land use (e.g. brush management) on biogeochemical processes which, are highly variable and otherwise difficult to measure at the regional scale.; Finally, an assessment of the ecosystem N status following woody encroachment was conducted. Differences in N associated with woody and herbaceous canopies—often used to assess the magnitude of changes attributed to woody encroachment—were readily distinguishable in the systems of low overall N status. These relative differences lessened as gross N quantity increased.; Nitrogen is considered to be the most limiting nutrient to plant production in terrestrial ecosystems world-wide. The concept that increased N inputs via biological N fixation by woody plants such as Prosopis could alter the long-term balance of N in an ecosystem relative to that required for plant production is not new. However, measurement of N inputs relative to plant N requirement is difficult, and in the context of recent woody expansion into grasslands, may not capture short-term changes in N dynamics that impact the long-term trajectories of ecosystem development. The results presented in this thesis provide an understanding of the impacts of woody encroachment on N dynamics, which potentially affect the long-term structure and function of arid and semi-arid ecosystems. (Abstract shortened by UMI.)